Add a new Attr method to testing.TB that emits a test attribute.
An attribute is an arbitrary key/value pair.
Fixes#43936
Change-Id: I7ef299efae41f2cf39f2dc61ad4cdd4c3975cdb6
Reviewed-on: https://go-review.googlesource.com/c/go/+/662437
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
Damien Neil